1. Field of the Invention
This invention is concerned with the catalytic cracking of hydrocarbons in the absence of added hydrogen. In particular, it is concerned with a method for sustaining or increasing the carbon monoxide combustion activity of zeolite cracking catalysts that contain, as combustion-promoter, a minute amount of metal such as platinum.
2. Description of the Prior Art
The catalytic cracking of petroleum hydrocarbons is conducted commercially on a very large scale utilizing either a moving bed process in which fairly large particles of catalyst are continuously circulated between a cracking zone and a regeneration zone, or a fluid bed process in which a much smaller particle size catalyst is circulated through the system in a fluidized state.
Since by far the most important process in current use is that in which the catalyst is fluidized, commonly referred to as fluid catalytic cracking or "FCC," the present invention will be exemplified by reference to this process although it is to be understood that it is also applicable to the moving bed process and other conceivable processes of the same type. In the fluid catalytic cracking process, the installed plants are usually designed to process from about 5,000 to 135,000 bbls/day of fresh feed. Most of the plants are quite large, and are capable of handling at least 40,000 bbls/day. The catalyst section of the plant consists of a cracking section where a heavy hydrocarbon feed is cracked in contact with fluidized cracking catalyst and a regenerator section where fluidized catalyst coked in the cracking operation is regenerated by burning with air. All of the plants utilize a large circulating inventory of cracking catalyst which is continuously circulating between the cracking and regenerator sections. The size of this circulating inventory in most existing plants is within the range of 50 to 600 tons.